At this point, members of the public have probably heard the following mantra a hundred times: while omicron is more infectious than delta, it also produces a less severe infection. This has been repeated over and over since the variant was first introduced to the public by a group of South African scientists.
Well, just in case any doubters remain, the NYT reported the results of the latest round of studies on New Year's Day. The studies mostly focused on animals: In studies on mice and hamsters, omicron produced less damaging infections, often limited largely to the upper airway: the nose, throat and windpipe.
Most important: omicron appears to do less harm to the lungs, where previous variants would often cause scarring and lead to serious difficulty breathing for many patients. But the animal studies show that omicron typically stays in the windpipe and upper respiratory tract: it doesn't make its way down deep into the lungs like delta.
"It’s fair to say that the idea of a disease that manifests itself primarily in the upper respiratory system is emerging," said Roland Eils, a computational biologist at the Berlin Institute of Health, who has studied how coronaviruses infect the airway.
When omicron was first introduced back in November, the only thing scientists knew for certain was that it had more than 50 mutations, many involving the spike protein used by the virus to bind with human cells. But as scientists have discovered in the interim, there is more to a virus than its mutations.
"You can’t predict the behavior of virus from just the mutations," said Ravindra Gupta, a virologist at the University of Cambridge.
And after months of scientists' infecting cells in Petri dishes and spraying the virus into the noses of animals, scientists have learned a little more about omicron. Interestingly, some scientists found the virus behaved in interesting ways, with reactions in certain species of animals drawing attention:
Although the animals infected with Omicron on average experienced much milder symptoms, the scientists were particularly struck by the results in Syrian hamsters, a species known to get severely ill with all previous versions of the virus.
"This was surprising, since every other variant has robustly infected these hamsters," said Dr. Michael Diamond, a virologist at Washington University and a co-author of the study.
For now, scientists suspect that omicron's more mild demeanor might be a product of the human anatomy, moreso than the virus's genetic makeup.
The reason that Omicron is milder may be a matter of anatomy. Dr. Diamond and his colleagues found that the level of Omicron in the noses of the hamsters was the same as in animals infected with an earlier form of the coronavirus. But Omicron levels in the lungs were one-tenth or less of the level of other variants.
A similar finding came from researchers at the University of Hong Kong who studied bits of tissue taken from human airways during surgery. In 12 lung samples, the researchers found that Omicron grew more slowly than Delta and other variants did.
The researchers also infected tissue from the bronchi, the tubes in the upper chest that deliver air from the windpipe to the lungs. And inside of those bronchial cells, in the first two days after an infection, Omicron grew faster than Delta or the original coronavirus did.
Although others have found characteristics in human lung tissue that help to prevent the new variant from spreading in the lungs. Specifically, a protein called TMPRSS2 on the surface of the inside of the lungs. This protein doesn't take to omicron, impeding its spread in the critically important organ.
Many cells in the lung carry a protein called TMPRSS2 on their surface that can inadvertently help passing viruses gain entry to the cell. But Dr. Gupta’s team found that this protein doesn’t grab on to Omicron very well. As a result, Omicron does a worse job of infecting cells in this manner than Delta does. A team at the University of Glasgow independently came to the same conclusion.
Through an alternative route, coronaviruses can also slip into cells that don’t make TMPRSS2. Higher in the airway, cells tend not to carry the protein, which might explain the evidence that Omicron is found there more often than the lungs. Complicating matters, there are cells in the lungs that react to intruders by destroying all cells, not just infected ones.
Of course, more studies will need to be conducted before the scientific community can say anything for certain.
These findings will have to be followed up with further studies, such as experiments with monkeys or examination of the airways of people infected with Omicron.
If the results hold up to scrutiny, they might explain why people infected with Omicron seem less likely to be hospitalized than those with Delta.
Right now, this is all we can say for certain: COVID infections start in the nose, or possibly the mouth, before spreading down the throat. Mild infections don’t get much further than that...but when the virus takes hold in the lungs, it can then cause serious, lasting tissue damage.